Well, I contacted
support@aldiko.com with the problem. Meanwhile I downloaded Moon + and don't like it. I removed Aldiko from my Vox and tried installing it again from Google Play but got the same error again. I also tried installing older versions from the web, but they wouldn't install.
I got a reply from Aldiko with a Dropbox link to download v.3.0.35 which worked and I even got all my books back. Aldiko also sent me a link to a crash reporting app to use. So far Aldiko is working, but still crashes when I try to change the font. Maybe newer versions use more and more resources and the poor underpowered Kobo Vox just can't deliver. Maybe I should start deleting books I've read and apps I hardly use.